1. Field of the Invention
The present invention relates to a process for recovering crystals from a slurry which is used in processes for producing various organic chemical products such as a process for producing terephthalic acid by liquid phase oxidation of para-xylene.
2. Description of the Prior Art
When terephthalic acid is produced by liquid phase oxidation of para-xylene, formed terephthalic acid is crystallized in the mother liquor and a slurry containing crystals of terephthalic acid is formed. Crude terephthalic acid is obtained by recovering the crystals from the slurry. When the obtained crude terephthalic acid is dissolved and purified by treatments for purification such as oxidation treatments and reduction treatments and the purified terephthalic acid is crystallized, a slurry containing crystals can be obtained.
In both treatments of a slurry described above, a rotary vacuum filter (referred to as RVF hereinafter) which run successive filtration under suction, washing, filtration under suction and cleavage of a cake in a batch operation using is most frequently conducted as the process for recovering the crystals from the slurry.
In the operation of RVF, filtration under suction, washing, filtration under suction and cleavage of a cake are conducted successively while a cylindrical filter medium is rotated. However, a continuous operation for a long period of time is difficult in the operation of RVF since the filter medium is clogged during the continuous operation.
The present invention has an object of proposing a process which enables recovery of crystals continuously for a long period of time without clogging of a filter medium when the crystals are recovered from a slurry by using RVF.
As the result of intensive studies by the present inventors on RVF having the above problem, it was found that cleavage of a cake can be facilitated by supplying a vapor having the same composition as that of a solvent as the blowing gas used for cleavage of the cake in the step of cleavage by RVF and the operation of RVF can be continued for a long period of time. The present invention has been completed based on the knowledge.
The present invention provides a process for recovering crystals from a slurry which comprises supplying a slurry comprising crystals and a solvent to a rotary vacuum filter and successively conducting filtration under suction, washing, filtration under suction and cleavage of a cake continuously by rotation of a cylindrical filter medium, wherein a vapor having a same composition as that of the solvent is supplied as a blowing gas for cleaving the cake.